Covalent Organic Framework with Multiple Redox Active Sites for High-Performance Aqueous Calcium Ion Batteries

Organic materials are promising for cation storage in calcium ion batteries (CIBs). However, the high solubility of organic materials in an electrolyte and low electronic conductivity remain the key challenges for high-performance CIBs. Herein, a nitrogen-rich covalent organic framework with multiple carbonyls (TB-COF) is designed as an aqueous anode to address those obstacles. TB-COF demonstrates a high reversible capacity of 253 mAh g^-1 at 1.0 A g^-1 and long cycle life (0.01% capacity decay per cycle at 5 A g^-1 after 3000 cycles). The redox mechanism of Ca²⁺/H⁺ co-intercalated in COF and chelating with C═O and C═N active sites is validated. In addition, a novel C═C active site was identified for Ca²⁺ ion storage. Both computational and empirical results reveal that per TB-COF repetitive unit, up to nine Ca²⁺ ions are stored after three staggered intercalation steps, involving three distinct Ca²⁺ ion storage sites. Finally, the evolution process of radical intermediates further elucidates the C═C reaction mechanism.